Short duration power variations on a MW scale occur in many different systems. For example, electrified rail experiences voltage surges and sags when trains decelerate and accelerate. Areas of the grid experience surges, sags and ramps when variable renewable energy sources, particularly wind and solar, provide a significant fraction of the generated power. Draglines used at mines, material handling equipment, and islanded systems all experience frequent, short duration MW scale power fluctuations. Heretofore, the high power and high number of cycles of these disturbances has made it prohibitive for conventional energy storage solutions to be applied. Instead, the excessive fluctuations are dealt with by ramping generating assets with the consequence of reduced efficiency, higher energy cost, higher O&M cost, and reduced service life.
A more effective and less costly solution to this problem is an energy storage device that can source or sink 1 MW for approximately 90 seconds, switch between charge and discharge in a few milliseconds, cycle continuously, and deliver 1,000,000 or more full charge-discharge cycles without degradation. In order to be of value in a range of applications, the storage system should be relatively compact and transportable so it may be deployed in many applications.
No battery or capacitor can economically provide this capability without periodic replacement or significant oversizing. Pumped hydro storage cannot respond as rapidly and is neither compact nor transportable. Heretofore no flywheel energy storage system has attained all of these capabilities including power and stored energy.
Batteries are used in short duration power management applications such as hybrid vehicles. However, even the most durable batteries have a throughput capacity of 3000 which corresponds to 3000 cycles with a depth of discharge of 100% of the capacity of the battery. Consequently batteries are useful in applications with limited cycle life such as vehicles but cannot withstand long-term operation with frequent cycling. Ultra-capacitors have much greater cycle life than batteries but degrade after several hundred thousand cycles and are costly when sized to store more than a few seconds worth of energy.
Flywheels are well suited to withstanding large numbers of cycles without degradation or failure. Cycle life is limited by the fatigue strength of the material that at 1 million cycles is about 50% of new strength for steel and 85% of new strength for carbon composite. However, operation with continuous high power cycling has proven challenging. Even small inefficiencies resulting in losses on the rotor can cause excessive rotor heating. Heretofore no flywheel system with the ability to cycle continuously at more than approximately 200 kW has been developed.